Steel of multi-purpose application is a term used in reference to the steel of the following composition, % by mass:
______________________________________ carbon 0.05-0.5 manganese 0.25-2 iron the balance. ______________________________________
Steel of multi-purpose application may also contain (% by mass) such elements as:
______________________________________ silicon up to 0.6 aluminium up to 0.08 chromium up to 2 vanadium up to 0.2 titanium up to 0.2. ______________________________________
The presence of other elements is not excluded as well.
In world steelmaking practice, confined to the ladle are now the following operations: alloying, desulphurisation, modification and the removal on nonmetallic inclusions, degassing, i.e. the reduction of oxygen, nitrogen and hydrogen content.
The alloying and refining are carried out in succession. The steel is alloyed by adding various ferroalloys and then it is refined by such techniques as the application of vacuum or the introduction of powdered material with the aid of a jet of an inert gas.
Considerable heat losses are involved during each operation of alloying and refining. To compensate for the losses, the metal must be overheated in the furnace and tapped at a temperature above normal or it must be heated up in the ladle, using certain means.
However, both ways are irrational in the making of steel for multi-purpose application. Firstly, an increase in the solubility of oxygen at high temperatures results in a higher than normal oxidation of the metal. This calls for using more deoxidisers which contaminate the steel by the nonmetallic inclusions resulting from the reaction and impair product quality. Secondly, the extra heating up with special means adds to the costs, for such means must be purchased and installed, and also extends the period of treatment, reducing plant capacity.
All in all, the cost of steel rises--a fact which is not justifiable as far as the production of steel for multi-purpose application is concerned.
Known in the art is a material for the refining of molten metal (EP, A1, 0192090) which has a composition (% by mass) as follows:
______________________________________ silicon 40-80 titanium 10-20 magnesium 1.5-3 calcium 0-0.5 aluminium 0-2 rare-earth elements 0-2 iron the balance. ______________________________________
However, the known material is ineffective in removing sulphur and nonmetallic inclusions, for the percentage of the oxygen dissolved in the molten metal is low.
The content of reactive agents, i.e. deoxidisers--aluminium, magnesium, calcium and rare-earth elements--which are also strong desulphurising additives and strong modifiers of nonmetallic inclusions, is low in the known material. The available deoxidisers effectively eliminate the oxygen dissolved in the molten metal when the known material is added thereto but they appear to be in short supply for the desulphurisation and modification to take place.
Apart from that, the titanium which is present in the known material for refining forms high-melting oxides. Rising to the surface, these oxides render the slag more viscous and less effective as the sorbent of sulphur and nonmetallic inclusions. These unwanted substances remain in the metal, impairing the quality thereof.
The known refining material, if used as the source of alloying elements reduced form oxides contained therein, is of no avail in producing metal of a conditioned composition. The silicon contained in the known refining material has a high affinity for oxygen but it is also a reducing agent of a strength inferior to that of aluminium, magnesium and calcium. Therefore, the reaction yields acidic oxides of silicon which increase the viscosity of the slag and reduce the reactivity of the alloying element contained in the slag. This has an adverse effect on the reduction of alloying element. Also, the process of desulphurisation is difficult due to an impaired ability of the slag to remove sulphides and act as the sorbent of nonmetallic inclusions.
All the above factors spoil the quality of steel. Also known is a material for refining (SU, A, 456,032) of the following composition, % by mass:
______________________________________ manganese 48-60 silicon 28-32 aluminium 6-12 calcium 0.4-3 magnesium 0.3-2 carbon 0.06-0.3 phosphorus 0.04-0.35 sulphur 0.01-0.02 iron the balance. ______________________________________
However, this material cannot boast good results as far as the degree of desulphurisation and removal of nonmetallic inclusions is concerned.
The explanation is the qualitative and quantitative composition of the known material. In the first place, the percentage of the elements with a high affinity for oxygen--such as calcium, magnesium, aluminium and silicon--is low.
In the second place, it contains phosphorus and sulphur which are unwanted admixtures.
In the third place, there is an abundance of manganese which reacts with the sulphur to form a low-melting manganous sulphide--a substance which readily dissolves in the molten metal and interfers with the slagging of sulphur.
In the fourth place, the carbon is present in the known material as an admixture so that extra carbon is required for refining the metal. This involves an increase in the sulphur content of the metal and a degrading of product quality.
The use of the known material as an oxide-containing alloying additive is impractical. Although, there are elements which eagerly react with the oxygen of oxides, their percentage is low. The degree of reduction is consequently low as well so that no steel of specified composition can be produced. Desulphurisation and the removal of other nonmetallic inclusions are totally absent so that no quality stell can be made.